U.S. forces reportedly came under artillery fire from Turkish troops heading into northern Syria last week — another sign of the sudden plunge in U.S. relations with Turkey.

On Monday, President Trump imposed economic sanctions against Turkey and threatened to “swiftly destroy Turkey’s economy.” Vice President Pence announced a cease-fire agreement with Turkey on Thursday, but this does not appear to fully address the underlying problems in the bilateral relationship. Over the summer, the Pentagon kicked Turkey out of the F-35 joint strike fighter program, marking U.S. displeasure that Turkey was buying advanced Russian military technology.

This deteriorating relationship is troubling because Turkey is a long-standing NATO ally. But even more worrisome are the nuclear weapons — about 50 B61 gravity bombs — that the United States stores at Incirlik Air Base in Turkey, less than 100 miles from the Syrian border. On Wednesday, Trump appeared to confirm the existence of these weapons in a startling break with past practice, but over the weekend, U.S. officials reportedly were considering plans to withdraw them.